<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 100px;
            padding: 20px;
            border: 5px solid black;
            margin: 20px;
        }
        .border-box{
            /* border padding 先分配，剩余的是内容大小
               多列布局的时候有优势
            */
            background-color: lightblue;
            box-sizing: border-box;
        }
        .content-box{
            background-color: lightgreen;
            box-sizing: content-box;
        }
    </style>
</head>
<body>
    <div class="box border-box">Border with border-box</div>
    <div class="box content-box">Box with content-box</div>
</body>
</html>